PCX

Image file format that use palleted colors and simple [run length encoding]? lossless compresion algoritm. PCX was quite popular on early DOS and Windows systems, but it nowadays very rare, replaced by formats which support much better compresion and full 24-bit images, like GIF, JPEG and PNG.

PCX compresion

Decompression algorithm is very simple. Every byte from range 0x00 to 0xc0 encodes single pixel of this value. Bytes from range 0xc1 to 0xff mean that next 1 to 63 pixels have the same color, which is encoded in the next byte. Colors 0xc1 to 0xff are encoded using length 1 prefix followed by their code: 0xc1 COLOR.

Because colors 0x00..0xc0 are compressed better than colors 0xc1..0xff, good pallete sorting is important. It's usually (but not allways) enough to move most-common colors into palette positions 0x00..0xc0, and least-used to palette positions 0xc1..0xff.

This compresion algorithm is very fast and takes very little memory, but its not very efficient, especially in compressing real-world images.